For decades, astronomers have puzzled as to why a bright, heart shaped region on Pluto, known as Tombaugh Regio, appears to be rotationally locked to face the dwarf planet's largest moon, Charon. Data collected by NASA's New Horizons probe during its July 2015 flyby of the dwarf planet may have finally solved the riddle, by supporting the hypothesis that a large, slushy subsurface ocean exists just beneath the surface of Tombaugh Regio.

"People had considered whether you could get a subsurface layer of water somewhere on Pluto" states New Horizons co-investigator Richard Binzel, professor of earth, atmospheric and planetary sciences at MIT. "What's surprising is that we would have any information from a flyby that would give a compelling argument as to why there might be a subsurface ocean there. Pluto just continues to surprise us."

According to New Horizons' science team, the presence of this theoretical ocean, and of the gravitational lock between Tombaugh Regio and Charon, is the result of a collision that took place many millions of years in the past between Pluto and a large impactor.

This collision is thought to have carved out a large basin in what we now know as Sputnik Planitia, the enigmatic region that comprises the left "ventricle" of Tombaugh Regio's heart. Images of the alien landscape obtained by New Horizons revealed that the surface of Sputnik Planitia is divided into cell-like formations composed of nitrogen-rich ice.

Previous studies of New Horizons' data have suggested that the surface of Sputnik Planitia is constantly refreshing itself. Through a process known as convection, cooler nitrogen ice located in the middle of the cells falls away, and is replaced by a rising globule of (relatively) warm material, which has been heated by Pluto's feeble interior heat source.

The fact that heat is able to escape Pluto's interior to warm the ice suggests that the impactor strike may have thinned the crust below Sputnik Planitia, creating a weak spot through which the heat could rise.

In order to determine how this interaction could have induced Tombaugh Regio to become rotationally locked with Charon, the team ran simulations using a sophisticated geophysical model of Pluto's interior, constructed with the help of data harvested by New Horizons.

Taking into account the dwarf planet's size and interior heat flow, which is around two percent that of Earth's, the team discovered that the temperatures and pressures at play below Sputnik Planitia could give rise to a viscous, slushy subsurface ocean of water ice.

Over the course of millions of years, the gravitational weight of this dense ocean could have been enough to manipulate the rotation of the planet, and orientate Sputnik Planitia to face Charon.

Sputnik Planitia's location at the equator of Pluto could also have played an important part in maintaining the locked orientation between the subsurface ocean and Charon. Data from New Horizons suggests that Pluto's polar regions experience long winters, and (at least for Pluto) hot summers. However, the dwarf planet's equatorial regions maintain a moderate temperature, thanks to the more regular transitional periods between night and day.

Because of this, ice deposits in the polar regions are likely to be diminished in the summer period, while the build-up of ice in Tombaugh Regio may have endured for millions of years. The team believes that the consistent mass of the ice sheets, combined with the gravitational weight of the subsurface ocean beneath, could finally put to rest one of Pluto's many mysteries.

A paper on the research has been published online in the online journal Nature.